The smelting of concentrates of copper, zinc, lead, nickel, cobalt, silver or gold is accompanied by the volatilization of arsenic that is very often a minor but significant component of sulfide concentrate. This volatilized arsenic, under the form of arsenious oxide (As2O3), is condensed and collected by various approaches, such as electrostatic precipitation, wet scrubbing or filtration through bag houses. The volume of arsenic thus produced far exceeds the market demands, and most of it has to be disposed of in a safe manner.
One technique largely used is the formation of ferric arsenate (scorodite: FeAsO4.2H2O) from the arsenious oxide. A solution of As2O3 in water is oxidized to As2O5 with an oxidizing agent such as chlorine or hydrogen peroxide, and this solution is contacted with a ferric salt at a proper pH, with a large excess of iron, up to eight moles of iron per mole of arsenic (N. Papassiopi et al., Hydrometallurgy 41, 243-253 (1996)). Under these conditions, very low solubilities of arsenic are observed, but the presence of other ions, particularly sulfates, can interfere with these results. Large iron consumption, along with the use of expensive reagents (Cl2, H2O2), and delicate operational conditions, make this technique costly.
Other approaches have been used to remove arsenious oxide from solutions, either precipitation, membrane and adsorption (E. O. Kartinen et al., Desalination 103 19-88 (1995)). In all cases, the results were significantly below performances reported for the scorodite method.
In order to obtain a sequestration of arsenic that would be more permanent than scorodite, several attempts of vitrification of arsenic have been reported (Arsenic Treatment Technologies for Soil, Waste and Water: EPA-542-R-02-004, September 2002, 5-1 to 5-8). These approaches go from arc heating of the arsenic-bearing materials in the soil to treatment of particulates loaded with arsenic in process effluents. In all cases, the arsenic content was at most 3 percent of the mass treated, the low solubility of arsenious oxide in silica glass being the limiting factor. It was noted that this approach suffered from several limitations, such as important volatilization of the arsenic in the process, interference of anion such as chlorides, fluorides and sulfates with the quality of the glass, glass solubility and toxic volatiles formation, such as dioxins.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.